I have a combo of both where it's vague but not so vague. I leave plotting for figuring out the flow of the story and pantsing for the actual panels/dialogue.
My scripts are usually summaries of each chapter, told in simple prose. The only details I might go into are around emotions, character interactions, and maybe some dialogue. I mostly write the concrete ending first before going into the beginning and middle.
However, I leave it vague enough so that I can pants when it comes to making thumbnails.
If I have a scene like "Johnny went to the store to by apples. He saw Jill, and they had a fight about their breakup." -- I've left plenty o' room to freely add and drop moments for this scene. I can improvise dialogue, emotions, and gestures.